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Respiration Ventilation: air entering and leaving the alveoli. Transfer through the alveolo- capillary membrane : exchange of O 2 and CO 2 between the blood in the capillaries and the gases in the alveoli. Perfusion - Circulation : transfer of O 2 from the pulmonary capillaries to the cells of the body and transfer of CO 2 from the cells to the pulmonary capillaries. Cellular respiration : transfer of O 2 in the blood to the cells and CO 2 from the cells to the blood. PHYSIOLOGY

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Oxygenation Oxygen : –Minute ventilation is the amount of fresh gas delivered to the alveolus –Partial pressure of oxygen in alveolus (P A O 2 ) is the driving pressure for gas exchange across the alveolar-capillary barrier –P A O 2 = ({Atmospheric pressure - water vapor}*FiO 2 ) - P a CO 2 / RQ –Match perfusion to alveoli that are well ventilated –Hemoglobin is fully saturated 1/3 of the way thru the capillary

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Abnormal Gas Exchange Hypoxemia can be due to: –hypoventilation –V/Q mismatch –shunt –diffusion impairments Hypercarbia can be due to: –hypoventilation –V/Q mismatch Due to differences between oxygen and CO 2 in their solubility and respective disassociation curves, shunt and diffusion impairments do not result in hypercarbia

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Gas Exchange Hypoventilation and V/Q mismatch are the most common causes of abnormal gas exchange in the PICU Can correct hypoventilation by increasing minute ventilation Can correct V/Q mismatch by increasing amount of lung that is ventilated or by improving perfusion to those areas that are ventilated